1. Field of the Invention
The present invention relates to an apparatus for analyzing biological liquids, and especially of the type by means of which it is possible to make very accurate measurements on small liquid samples. In order to obtain the desired accuracy in measurement it is often necessary to secure that the liquid samples which are fed into the apparatus as well as the measuring means of the apparatus are maintained at a substantially constant temperature during measurement.
2. Description of the Prior Art
For the above reason it is usual to arrange the measuring means of the apparatus within a thermally insulated housing and to heat the measuring means to the desired constant temperature by means of a flowing medium, such as air or water, the temperature of which is thermostatically controlled, and the measuring means of the apparatus may possibly be mounted within a block or a measuring body having flow passages formed therein for the liquid samples to be measured, cleaning liquids, salt bridge liquids, etc.
However, the use of such flowing heating mediums involves the use of pumps, blowers, or similar medium circulating means which are undesired because such circulating means increase the complexity of the structure.
U.S. Pat. No. 3,811,493 discloses a thermal shield consisting of a pair of walls defining a space which contains a two-phase heat exchange fluid. The said thermal shield, which is adapted to enclose the upper housing of a gyroscope, is heated by an electric heater, which is arranged at the outer surface of the shield, and controlled by a temperature sensing device mounted on the said upper housing of the gyroscope.
U.S. Pat. No. 3,246,688 discloses an apparatus comprising a centrifuge arranged within an evacuated housing, which may be heated by means of a heating device, the operation of which is controlled by temperature sensors arranged immediately adjacent to the rotating centrifuge and on the said housing, respectively.
It is noted that in the known thermal shields described above a thermostatically controlled heating of the shielding housing itself takes place. If a medium with a temperature differing from that of the shielding housing was continuously or intermittently fed into a device arranged within such a housing, transmission of heat from the shielding housing to the apparatus arranged therein would occur substantially by convection and/or radiation with resulting temperature gradients in the direction from the housing to the apparatus enclosed thereby.
When an apparatus for analyzing biological fluids is in use, liquid samples and possibly also salt bridge liquids, cleaning liquids, and other media are continually fed to the measuring means of the apparatus. Thus, if the temperature control means are adjusted so as to maintain the thermally shielding housing at a temperature above the ambient temperature--as is normally the case--the liquid supplied cause a constant cooling of the measuring means. Therefore, if such a measuring apparatus was arranged in a thermal shield of the type disclosed in the above mentioned U.S. patent a constant transmission of heat from the heated housing to the measuring means arranged therein would take place constantly, and such heat transmission would take place substantially by convection and/or radiation which would give rise to a high thermal time constant between the generator of heat and the temperature sensor.
The prior art also comprises temperature controlled apparatus as those disclosed in U.S. Pat. No. 3,763,422, British Pat. No. 1,488,708, German Auslegeschrift No. 2,039,433, and German Offenlegungsschrift No. 1,523,396.